This invention relates generally to the field of electric heating and specifically to a controller for multi-phase heating panels.
Ovens and cooktops are commonly heated by one or more of several means, including burning combustible gases and electrical resistance. One form of electrical resistance heating uses monolithic integrated heat sources, known as "heat panels," disposed on walls of the oven or on the cooking surface of the cooktop. Heat panels include a thermally and electrically conductive metal substrate or core covered on one or both faces by an electrically insulative material able to conduct heat. One face of the insulative material has a heating layer or film of electrically resistive material disposed thereon and connected to a current to generate heat. Alternatively, the heating layer is disposed on a glass ceramic substrate. The glass ceramic is crystallized glass that can be used as the cooktop. The heat is conducted from the heating layer through the other layers to the oven cavity or cooking vessel on the cooktop. Examples of such apparatus are shown in U.S. Pat. Nos. 4,298,789 to Eichelberger and 5,577,158 to Kallgren and application Ser. No. 08/503,039 filed Jul. 17, 1995 by Kallgren, et al, incorporated herein by reference.
Industry standards sometimes require the substrate to be connected to ground. The electrically conductive layers separated by an insulating layer form a capacitor. Thus, when an alternating current (AC) passes through the heating layer, a capacitive AC leakage current caused in one conductor, such as the substrate, and a resistive leakage current through the insulator become leakage current to ground when the substrate is connected to ground. The leakage current to ground will usually exceed industry standards or codes. In addition, if the substrate is connected to neutral or ground of a power source, the leakage current should also be minimized to the power source. If the substrate is floating, the electrical potential that builds up on the substrate should be minimized.